After posting an example of the Drupal form_alter hook, I thought it could be useful to post a list of places where you can find extra information about the Drupal forms system.
Forms
Bonus Forms: A useful collection of Drupal forms information
Drupal Forms – The form_alter() Hook
What is the form_alter hook?
After following through my first tutorial on forms, you should be able to create a form, check that the input is as you require, and submit the data for processing. But what if you want to add functionality to certain areas of Drupal that you do not control, like contributed modules, or even Core?
Drupal Forms - Your First Form
So you need a Form. Where do you start?
If you're anything like me (which I hope you are, because you're reading this!), then you'll want to look at the Drupal Forms API.